The present invention relates generally to constant current driver circuits. More particularly, this invention pertains to improved buck converters for driving light emitting diodes (LEDs) to emit light from a light fixture.
Compared to incandescent lamps and fluorescent lamps, LED lighting has a longer useful life and superior dimming capability. That is, dimming an LED light source will not affect the life span of the LED light source. Thus, as the cost of LED lighting decreases, LED lighting is becoming the lighting of choice for most applications.
Referring to FIG. 1, an LED light source 104 needs a constant current driver circuit to provide a consistent level of light output. A buck converter 100 is the basis for most constant current LED driver circuits. The switch 102 (e.g., a MOSFET) in the conventional buck converter 100 is on the high side floating. A controller 106 of the converter 100 is typically an average current control integrated circuit that needs to be referred to the ground of the converter to sense the average current through the load (i.e., the LED light source 104). Thus, an isolation transformer is required for the controller 106 to turn the switch 102 on and off. The primary winding 108 of the isolation transformer is connected between a gate drive output of the controller 106 and a ground of the converter 100. The secondary winding 110 of the isolation transformer is connected between a control terminal of the switch 102 and a low side of the switch 102. A high side of the switch 102 is connected to a positive input of the converter 100. Eliminating the isolation transformer would reduce the cost, size, and complexity of the converter 100.